Mixture and method for imparting a corrosion-resistant surface to aluminum, aluminumalloys, and silver



MIXTURE AND METHOD FOR IMPARTING A CORROSION-RESISTANT SURFACE T ALU- M'ENUM, ALUMINUM ALLOYS, AND SILVER Raymond Strlcklen, Baltimore, Md, assignor to Allied Research lrorlucts, lino, baltimore, Md., a corporation at Maryland New lllrnrving. Application July Z8, 1954 Serial No. 446,407

2! Claims. (Cl. 148-63) res PatcntO This application is a continuation-in-part of applicant's co-pending application, Serial No. 295,552, filed June 25, 1952, now abandoned, the latter being a con- 1 tinuation-in-part of applicant's application Serial No. 139,114, filed January 17, 1950, now abandoned.

in the case of aluminum, the coatings previously used commercially have been anodic oxide films produced by electrolysis in acid or alkaline solutions; oxide films produced chemically in alkaline oxidizing solutions; and phosphate coatings produced in acid phosphate solutions which may or may not have been modified by the prcsence of chromatcs.

The present invention provides equal or superior corrosion resistance to any of the above. It removes the necessity of high solution temperature and ventilation required in all of these prior treatment methods. it shortens the immersion time necessary to reach an equal degree oi protection. it removes the necessity for power supply and the individual racking oil parts necessary in the anodic processes. Furthermore, it is equally applicable to all alloys of aluminum in common use while the widely used chromic acid anodizing process is limited to those which contain l.5% or less of copper.

no additional advantage is the fact that all of the coatings of those processes previously used are electrical insulators while those obtained through this invention are electrical conductors. The electrical resistance at the present coatings is on the order of 1900 microhms 7 per square inch at a pressure of mu pounds per square inch which is negligible for most purposes. This has enabled manufacturers of electrical equipment to protect aluminum surfaces from corrosion and to make electrical contact with the some suriaccs without removing the protective coating. liesulls include lowered manufacturing costs and new flexibility of design possibilities.

The coating of the present invention has also been found advantageous in preparing aluminum for assembly by means of structural adhesives and has proved successful in bonding rubber to aluminum surfaces. In addition, it has been found to produce exceptionally durable bonds for paints and similar organic finishes.

in the assembly oil aluminum parts by the inert gas shielded arc welding method it was previously necessary to weld immediately other mechanical or chemical clean ing of the metal. A delay oil as much as 2d hours allows reformation of the natura prides on the aluminum and these inter-lore with lg optimum meld character-rio lies. When the aluminum parts are treated by this in vcution utter cleaning, they may be stored as long as sevoral months and still be welded without removal the coating cannot ftlhlll ll cleaning. Furtlren lllib more, the welds produced in metal treated by this invention are found to be of superior quality. This property enables manufacturers to operate separate schedules for their cleaning and welding operations and to make use of smaller equipment than would be necessary for treating completed assemblies. After welding the weld beads are cleaned by wire brushing or equivalent and then treated by brush application of the solutions described in this invention, thereby providing protection for the entire assembly.

A further property of the coatings of this invention is their ability to absorb dyes. While still wet, the coatlogs are immersed in a solution of an organic dye and thereby colored in a manner permitting ready identification in color code systems andthc like.

Aluminum alloys treated with the solutions of this invcntion for a period of 10 to 30 seconds are coated with a clear invisible film having appreciable corrosion pro tection. The protection is apparent on subjecting treated and untreated specimens to corrosive conditions such as the salt spray test described in Federal specification QQ-M-lSl. This is the first commercial process of the chemical immersion type to produce this type of protcctive clear film on alloys containing a high percentage ol silicon.

Treatment of 30 seconds or more produces a yellow this has no adverse effect on corrosion protection, the appearance may be undesirable for some applications.

The film formed by this process also exhibits the selfhealing characteristics of other chromatc films in protecting abraded areas.

Regarding silver and alloys containing silver as a primary constituent, an immersion of 10 seconds to 2 minutes produces a film of a pale yellow color which is barre ly visible. It is difficult to see under some conditions unless it treated piece is compared directly with an urn treated piece.

The coating attained on silver has a high degree oil tarnish resistance, being comparable with water dip lacquers which are widely used in tarnish protection on silver plate for industrial applications.

Its electrical contact resistance is on the order oli (ill to microhms per square inch of surface under a pressure of 100 pounds per square inch. This value is said to be negligible for most applications.

Silver surfaces which have been treated remain solderable under conditions which would tarnish untreated silver to such an extent that soldering is prevented.

The invention may comprise an acid chromatc solution or a dry powder mixture which may be dissolved in water to form such a solution. The solution may contain chromic acid 1 to 15 grams per liter, sodium fluosilicate l (silicoiluoridc) 5 grams per liter to 25 grams per liter or to saturation whichever is lower and boric acid 1 gram per the solution will be well on the acid side, namely not above 4.0 and usually between about pill 2.53 topll 1.3.

Patented July 15,

3, A. lower limit of pill 0.8 is contemplated but not preferred. it the pll of the solution is not below 2.5. small quantities of a mineral acid, preferably nitric. are added to ad ust the pli. Other mineral acids that may be used include any of the commercial grades oi sulfurlqchromic, fluosilicic and fluoboric acid. in operation, the solution is main- GJl. Gill 2.5 to 7 Na SiP 4.0 to 12 ii'iigBQg 4.0 to 12 are made by dissolving a mixture of dry powdered materials in water. The dry powdered mixture is used primarily for its low cost in packaging and shipping although a liquid concentrate could be would serve the purpose.

The preferred dry mixture is as tollows:

prepared which Percent by weight CE'Ug il flgslFu H 80, 38.5

It may be dissolved in a concentration between 10 and 85 g./l. of solution. A solution of about 22.5 gJl. (3.0 oz./gal.) gives a solution of the following composition.

Example l 6.. 3 5.2 Nfi SlF 8.65 agao, ass

A solution with a pH of about 1.6 is generally obtained. if the pH is above 1.8 it is lowered by small additions of chromic, liuosilicic, iiuoboric, sulfuric, or nitric acids. The latter is preferred.

ll the pit of the solution is below l5 it may act too rapidly for the mechanical limitations or the work handling facilities. ll desired the pill may be raised by small addi lions oi Nomi or Nil llli. Either is satisfactory. The pill is best measured by means of a glass electrode and clcctrorneter. The concentration or? the solution is determined by iotlerntric titration.

The solution of l is preferred ior aluminum parts which are to be treated by immersion or by brushing; and for silver plated parts which are handled in built. "the results obtained by utilizing the iorcgoing example quite favorable and produced a corroslon resistant coating: on aluminum and a tnrnislnresistnnt coating on silver, tully discussed heretofore. 'llnc immersion time tor the. loregoing was about l seconds and the tempera ture was about @ti course, those conditions can be varied as noted in the specification.

Example li This solution is made by dissolving the preferred A powder mixture at a concentradon of M23 gJl. (i tin/gallon).

on. on, an su ars, to H530,

won

aluminum in ill Pill The effectiveness, on aluminum, of the solutions heretofore disclosed may be improved by the addition of an inorganic chloride. Sodium chloride is preferred for its low cost but ammonium chloride, potassium chloride,

ferric chloride may be substituted in stoichlometrically equivalent concentrations.

' Example 111 A suitable working solution containing the preferred sodium chloride consists of:

G./l. CtO 4.9 Na,sn=, 8.1 H 80, 8.1 NaCl 3.9

The pH of such a solution is about 1.3. It may be adjusted as in previous examples.

Although the solution of Examples 1 and ll give ex cellent results, on aluminum, the use of the solution of Example III, which includes sodium chloride, yields definite improvements. For example, the resistance of the films resulting from immersion in said solution, to salt spray is increased by to 500%. In addition, it should be noted that the optimum pH range of the working solution is extended by the addition of the inorganic chloride from the previous pH 1.3 to pH 2.5 to a new range of pH 1.0 to pH 3.0. By working in the lower end of this new pH range (pH 1.0 to pH 1.3) the treatment time for a given degree of protection may be shortened by as much as 50 to when compared to the time necessitated in the utilization of the chloride free solution. The

temperature range when utilizing the solution of Example III is 20 C. to 35 C.

The concentrations of chromic acid, sodium fluosilicate (silicofiuoride) and boric acid may be within the ranges noted heretofore. When sodium chloride is utilized, as

in Example Hi, the concentration thereof may vary from 0.5 gram per liter to 10 grams per liter.

Instead of immersing the aluminum, aluminum alloy, or silver articles in the solution, other means of treatment may be used, e. g., spraying, flow coating or brushing the solution on the articles.

While chromic acid is preferred, water-soluble com acid as recited. Likewise, instead of boric acid, watersoluble boric acid salts, e. g., alkali metal borates capable of supplying boric acid in water may be used in amount to provide boric acid in equivalent amount to that set forth. Also, instead of the fluo compounds mentioned, other water-soluble iluo compounds, e. g., alkali metal or ammonium iluosilicates and iluoborates may be used in amount to liberate an equivalent amount of fluosllicic or fluoboric acid as indicated above.

Of course, it can be appreciated that difierent concentration ranges than those noted heretofore would be applicable to the use of salts of chromic acid, salts of boric acid, and other water-soluble fluosilicates and fiuoborates. These different concentration ranges can be calculated using the ranges noted heretofore with respect to chromic acid, boric acid and sodium tiuosilicate. in other words, the calculations should be such that the same amount of hexavalent chromium, hereto, and fluorine radical would be realized in the mixture being formed.

in addition, as stated above, instead of the sodium chloride mentioned in Example III, other water-soluble chlorides, e. g, potassium chloride, ammonium chloride.

i should such that the of chloride radical would be realised in the mixture being formed.

Examples of solutions utilizing compounds other than the preferred compounds are hereinafter set forth.

Results that were obtained compare favorably with the results of Example i.

Example IV I G./l. CrO, 4.9 NuBF 16.7 M 90 8.3

The pit of this solution is about l.6 adjusted it necessary by the acid additions described for Example I. Dthss conditions are as in Example I.

pH of solutions in Examples V-VIII was about 1.6. Other conditions were as noted in Example 1.

Example IX G./l. IJRQCTQUTZHQQ 9 ti ht}; l hla silis u l0 liiNO to adjust pH to l5.

iiirutttplu It ....s.. m 6 ll' iillgiaufi q'lfifl m l ltln fiii l0 to udjust pit to l5.

Uthcr conditions of Examples lit and X were as in iiz'numplc ll.

'lFillB exact mechanism of the i'ornnution oi the corro sion rcsistuutiilm is obscure. However, ll believe that the ioliowiug iuuctions are performed by the constituents.

(it the chromic acid or equivalent compound serves to supply the hcxnvalent chromium necessary in tilni I iiormatiou.

t2) "the fluosilicate or tinoborate serves to activate the metal surface and to dissolve a small amount of metal. The solution of the metal reduces and precipitates chromium on the metal suriace in a gel structure.

(3) The boric acid or borntes not as a butter, primarily at the tuctal'soiution interface. At the interface the pH oi the solution is higher thou, that in the main body or the solution. This condition is necessary for the precipitation oi the reduced chromium. it is in this range that the boric ucid is most ctiective in coutroliing the acidity. control has the chest oi improving the hurducss oi the prccipituted tri-valcut Y chromium him.

till

The novel principles of this invention are broader than the specific embodiments recited above, and rather than unduly extend this disclosure by attempting to list all the numerous modifications which have been conceived and reduced to practice during the course of this development, these novel features are substantially defined in the following claims.

I claim:

1. A dry powder mixture for use in an aqueous solution to impart a corrosion resistant coating to metals, consisting essentially of a chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, and a lino compound selected from the group consisting of water soluble fluosilicates and fluoborates, said chromic compound in amount being equivalent to from about 1 to 15 grams of chromic acid per liter of solution, said boric compound in amount being equivalent to from about 1 to 45 grams of boric acid per liter of solution, and said fluo compound in amount being equivalent to from about 5 to 25 grams of sodium fluosilicate per liter of solution.

2. An aqueous acidic solution to impart a corrosion resistant coating to metals, consisting essentially of water, a chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, a fluo compound selected from the group consisting of: water soluble fluosilicates and fluoborates, and a mineral acid, said chromic compound in amount being equivalent to from about t to 15 grams of chromic acid per liter of solution, said boric compound in amount being equivalent to from about I to 45 grams of boric acid per liter of solution, said tluo compound in amount being, equivalent to from about 5 to 25 grams of sodium tluosilicate per liter of solution, and the mineral acid being equivalent in amount to give the aqueous solution a pH not greater than 4.0 or lower than 0.8.

3. An aqueous solution according to claim 2 having a concentration of the constituents in amount of about 10 to grams per liter.

4. The method of treating aluminum, aluminum alloys, and silver, to produce thereon a corrosion resistant coating which comprises subjecting the same to an aqucous acidic solution consisting essentially of water, a chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, and a fiuo compound sclectcd from the group consisting of water soluble fluosilicates and lluoborates, said solution having at pll between about pH 3.0 and pH 1.3, and wherein the chromic compound is in amount equivalent to from about i to 15 grams of chromic acid per liter of solution, said boric compound in amount equivalent to from about it I to 45 grams of boric acid per liter of solution, and said fluo compound in amount equivalent to from about 5 to 25 grams of sodium fluosilicate per liter of solution, and

wherein the concentration of ingredients is from about l 10 to 85 grams per liter, and wherein the treatment is conducted with the solution at a temperature of about 20 C. to 35 C. for from about 10 seconds up to about 10 minutes.

5. The method of treating aluminum, aluminum alloys, and silver to produce thereon a corrosion resistant coating which comprises subjecting the same to an aqueous acidic solution consisting essentially of water, a chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, and a tluo compound selected from the group consisting of water soluble tluosilicatea" v and l'luoborates, said solution having a phi between shout l3 and 3.0. and wherein the chromic compound is present in amount equivalent to from about l to l grams of chromic acid per liter of solution. said bortc compound in amount equivalent to from about 1 to 45 grams of horic acid per liter oi solution. and said fluo compound in amount equivalent to from about 5 to 25 grams of sodium tluosilicate per liter of solution, and wherein the concentration of ingredients is about to 85 grams per liter and wherein the treatment is conducted with the solution at a temperature of about 20 C. to 35C. tor irom about it) seconds up to about 10 minutes, and wherein the said solution is adjusted with mineral acid to a pit? not greater than about 3.0 or less than about 1.3.

a. it dry powdermixture for use in aqueous solution to impart a corrosion resistant coating to metal, consisting essentially of a chromic compound selected from the group consisting of chromic acid and water soluble salts thereof. a boric compound selected from the group consisting of boric acid and water soluble salts thereof, it duo compound selected iirom the group consisting 0t water soluble lluosilicates and fluoborates, and an inorganic chloride selected from the group consisting of an alkali metal chloride, ammonium chloride and ferric chloride, said chromic compound and said fluo compound being present in coating-producing proportions.

7. A dry powder mixture according to claim 6 wherein said chromic compound is in amount equivalent to from about I to l5 grams of chromic acid per liter of solution, said boric compound in amount equivalent to from about 1 to 45 grams of horic acid per liter of solution, said lluo compound in amount equivalent to from about i 5 to 25 grams of sodium iluosilicatc per liter of solution,

and the inorganic chloride in amount equivalent to from about 0.5 to it) grains of sodium chloride per liter of solution.

3. A dry powder mixture according to claim 6 wherein said inorganic chloride is sodium chloride.

9. A dry powder mixture according to claim 6 wherein said inorganic chloride is potassium chloride. 7

10. A dry powder mixture according to claim 6 wherein said inorganic chloride is ammonium chloride.

ll. ft dry powder mixture according to claim 6 wherein said inorganic chloride is ferric chloride.

l2. An aqueous acidic solution to impart a corrosion resistant coating to metals, consisting essentially of water,

a chromic compound selected from the group consisting oi chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, a duo compound selected from the group consisting of water soluble tluo silicates and tluoborates, and an inorganic chloride selooted from the group consisting oi alkali metal chloride, ammonium chloride and terric chloride, said chromic tffOmfiot'ittti and said tluo compound being present in coatting-producing proportions. g

iii. An aqueous acidic solution according to claim l2 wherein the chromic compound is in amount equivalent to from about t to l5 grams oi chromic acid per liter of solution, said horic compound in amount equivalent to from about i to 45 grams of boric acid per liter of solution, said iluo compound in amount equivalent to from about to 25 grams oi sodium fiuosilicate per liter oi solution, and the inorganic chloride is present in amount equivalent to trons about 0.5 to til grants of n- 53 Germany M 6 19 9 v sodium chlorideper liter oi solution,

Kill

ous acidic solution consisting essentially of water, a

chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, a boric compound selected from the group consisting of boric acid and water soluble salts thereof, a fine compound selected from the group consisting of water soluble fluo-- silicates and fluoborates. and an inorganic chloride selected from the group consisting of an alkali metal chloride, ammonium chloride and ferric chloride, said chromic compound and said tluo compound being present in coating-producing proportions.

l7. A method according to claim 16 wherein the chromic compound is present in amount equivalent to from about l to 15 grams of chromic acid per liter of solution, said boric compound in amount equivalent to from about 1 to grams of boric acid per liter of solution. said lluo compound in amount equivalent to from about 5 to 25 grams of sodium fluosilicate per liter of soluticn, and the inorganic chloride is present in amount equivalent to from about 0.5 to 10 grams of sodium chloride per liter of solution. and wherein the solution is adjusted with mineral acid to a pH not greater than about 3 or less than ubout'pH 1.3.

18. A method according to claim 16 wherein the treatment is conducted with the solution at a temperature of about 20C. to 35 C.

19. An aqueous acidic solution to impart a corrosion resistant coating to aluminum, aluminum alloys, and silver, consisting essentially of water, chromic acid, boric acid and sodium fiuosilicate.

20. An aqueous acidic solution as claimed in claim 19 wherein said chromic acid is in amount of 5.2 grams per liter of solution, the boric acid in amount of 8.65 grams per liter of solution, and the fluosilicatc in amountof 8.65 grams per liter of solution.

21. An aqueous acid bath for coating aluminum and I alloys thereof in which aluminum is the predominant ingredient, of the type wherein the essential coatingproducing ingredients are hexavalent chromium and a soluble fluorine-bcaring compound in coating-producing proportions; containing, as an addition agent, a quantity of boron (calculated as boric acid) in excess of the amount required for reaction with all of the fluorine present.

References Cited in the tilt: of this patent UNITED STATES PATENTS FOREIGN PATENTS "Jai 

21. AN AQUEOUS ACID BATH FOR COATING ALUMINUM AND ALLOYS THEREOF IN WHICH ALUMINUM IS THE PREDOMINANT INGREDIENT, OF THE TYPE WHEREIN THE ESSENTIAL COATINGPRODUCING INGREDIENTS ARE HEXAVALENT CHROMIUM AND A SOLUBLE FLUORINE-BEARING COMPOUND IN COATING-PRODUCING PROPORTIONS; CONTAINING, AS AN ADDITION AGENT, A QUANTITY OF BORON (CALCULATED AS BORIC ACID) IN EXCESS OF THE AMOUNT REQUIRED FOR REACTION WITH ALL OF THE FLUORINE PRESENT. 